1. Field of the Invention
The present invention relates to III-V compound semiconductors and more particularly to a method for producing III-Nitride semiconductor substrate materials.
2. Prior Art
It would be considered common knowledge that for successful epitaxial growth, the proper preparation of a substrate surface is required. In general, the preparation consists of two major readily distinguishable steps. The first step is essentially similar for any substrate material and comprises lapping and polishing, annealing, degreasing and cleaning in organic and inorganic solvents. The second step is unique for every substrate material and can be regarded as a fine tuning of the substrate surface towards the material to be deposited. The present invention relates to the second step in preparation of the substrate for the epitaxial growth of Group-III nitride; herein referred to as III-Nitrides, compounds including gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), and their alloys comprising nitrogen and Group-III semiconductors. It is equally effective for the pretreatment of both native and foreign substrates that is specifically important for the nitride semiconductors because of severe deficiency of the native substrates.
The III-Nitrides semiconductors attract attention as an exclusive platform for optoelectronic and power electronic devices enabling a wide range of applications from everyday general lighting to long distance communications. The generation and transfer of high energy fluxes through the crystalline solids of materials such as III-Nitrides semiconductors escalate dissipation processes caused by structural defects. Consequently, even a slight decrease in defect density of the material promotes expeditious realization of its ultimate merit. Reduction of the defects in the epitaxial III-Nitride layers has become a central issue in the development of the next generation III-Nitride devices. Most notably, the vast majority of the defects are generated at the substrate-layer interface. Accordingly, in order to mitigate the adverse effects of crystalline defects, optimal treatment of the substrate upon which the III-Nitride layer is grown is considered to be the most feasible way to achieve exceptional quality epilayers.
In U.S. Pat. No. 6,528,394, a method of GaN film growth on a sapphire substrate after multi-step pretreatment was disclosed. The bare sapphire substrate is nitridated in a growth reactor with ammonia, then sequentially exposed to a gas mixture of NH3 and HCl, after that an additional nitridation is applied. In one embodiment of that invention, hydride vapor phase epitaxial (HVPE) growth of a GaN film on a pretreated sapphire substrate was described. As a result of the growth, a mirror-like GaN film with still easily defined slight surface roughness is deposited. As it becomes clear from that patent description, the film achieves comparatively low structural quality characterized by 400 arcsec Full Width at Half Maximum (FWHM) for (00.2) symmetric x-ray diffraction (XRD) rocking curve (RC) that corresponds to the upper half of 108 cm−2 dislocation density.
U.S. Pat. No. 6,086,673 discloses a method of pretreatment a substrate that includes, first, formation of a pretreatment layer on the substrate, and, second, exposing this layer to a gaseous thermo-chemically reactive environment. As an embodiment of that patent, the pretreatment layer of ZnO is deposited on a sapphire substrate and then subjected to a gaseous environment that includes hydrochloric acid (HCl) and ammonia (NH3) thermo-chemically reactive to ZnO. Then, an epitaxial layer of GaN is grown in an HVPE process on the pretreated surface. As a result of the pretreatment, structural quality of the GaN epilayer is improved but not too much and remains even for more than 70 μm layer at high 108 cm−2 dislocation density level. There is no indication in the patent description that the disclosed pretreatment positively influences the achieved morphology of the epilayer.
Another U.S. Pat. No. 7,473,570 discloses the growth of GaN layer with less dislocation density on a sapphire substrate that is treated using reaction precursors Cp2Mg and NH3. Subsequent GaN deposition is carried out on a GaN-based buffer layer formed on the substrate to build epitaxial structure between the substrate and the layer. In that patent, it is claimed that such treatment together with the buffer structure can reduce effectively the dislocation density, thereby, high-quality epitaxy can be gained and the uniformity in the distribution of the dislocations can be improved. While some structural improvement of the epitaxial GaN is indeed observed, the dislocation density achieved by the disclosed method is still far above 108 cm−2 level, which is unacceptable for some advanced device structures, such as laser diodes, for example.
Further, U.S. Pat. No. 6,700,179 discloses treatment of the surface of a substrate for GaN epitaxy with silicon (Si) using silane (SiH4) as a silicon precursor. Acting as an anti-surfactant, Si is thought to promote better crystallization of a subsequent epitaxial layer if its optimal amount in controllable configuration is deposited. But difficulties to effectively control the uniformity and intensity of the treatment result in poor reproducibility of the process and non-uniform stress distribution in epitaxial layers.
In the paper of K. Naniwae, S. Itoh, H. Amano, K. Itoh, K. Hiramatsu and I. Akasaki, Growth of single crystal GaN substrate using hydride vapor phase epitaxy (J. Crystal Growth 99 (1990) 381), treatment of the sapphire substrate by gallium chloride (GaCl) just before the growth of GaN is mentioned. It is found that the implication of the GaCl pretreatment improves nucleation conditions, reduces the pit density and makes overall quality of grown GaN epilayer better. But, reduction of such coarse imperfections, as pits, is accompanied with neither the apparent decrease in the dislocation density any lower than 108 cm−2 level nor the smoothing of the inter-pit epilayer surface.
In another paper of Y. S. Cho et al. (Y. S. Cho J. Jhin, E. K. Koh, Y. J. Park, E. K. Kim, G. Kim, S. K. Min and D. Byun, Improved Crystalline Quality of GaN by Substrate Ion Beam Pretreatment, Jpn. J. Appl. Phys. 41 (2002) p. 4299), ion-beam pretreatment of a sapphire substrate is carried out aiming for the reduction of strains and dislocations in MOCVD GaN epilayer to be grown on that substrate. The 800 eV a reactive N2+ and the 55 keV N+ ion beams are used from a normal ion implanter (N+-implantation). Pretreatment with both ion beams results in the formation of a disordered amorphous phase on the substrate surface. It is found that the ion beam pretreatment favors a decrease in the strain and defect states in the GaN epilayer. But, roughness as well as dislocation density of the epilayers still have to be reduced considerably to make them suitable for the next-generation devices.